A great variety of filter sectors are known as disclosed, for instance, by GB-A-894 318, DE-B-24 07 218, U.S. Pat. No. 3,917,534, U.S. Pat. No. 3,971,722, U.S. Pat. No. 4,686,040 and U.S. Pat. No. 931,179. In operation, such filter sectors are exposed to large external pressures tending to press the plate shaped elements towards each other.
According to GB-A-894 318, in order to prevent this and also otherwise stiffen a filter sector, a plurality of spacing means is inserted between two plate-shaped elements of a laminated material impregnated with synthetic resin, said spacing means being kept in place by rivets or the like, which keep the two plate-shaped elements together.
For the same purpose, according to DE-B-24 07 218, the two plate-shaped elements are internally provided with integrated corresponding, radially directed support cams abutting each other and penetrated by screws keeping the two plate-shaped elements, which are cast from metal, together.
In the filter sector according to U.S. Pat. No. 3,917,534, the plate-shaped elements are formed in one piece of a plastic material. Solid posts integrated with the elements and keeping the elements apart extend between the elements. Further, the space between the elements is filled with a hardening, foam plastic material which further prevents compression of the elements and provides the entire sector with increased stiffness.
U.S. Pat. No. 3,971,722 discloses a filter sector comprising two identical plate-shaped elements formed from a plastic material, preferably polyurethane. On the insides of the elements are integrated solid spacing means which, when mounting the two elements, are connected to each other in order to keep the elements apart and together against external and internal pressures, respectively.
In the filter sector according to U.S. Pat. No. 4,686,040, a plurality of ribs extend substantially perpendicularly to the radial direction and is arranged between the plate-shaped elements which are made of pressed metal plate which is thin in comparison to the thickness of the filter sector.
According to U.S. Pat. No. 4,931,179 the space between the two plate-shaped elements, which are made of pressed metal plate which is thin in comparison to the thickness of a filter sector, is filled with a hardened compound adhering to the plate-shaped elements and keeping them spaced from each other while providing the filter sector with increased rigidity against distortion.
Some of the solutions mentioned require significant work to be performed when mounting a filter sector, such as making holes, fitting of spacers and riveting of the rivets thereof (GB-A-894 318), making holes, fitting of screws and tightening thereof (DE-B-24 07 218), fitting of ribs and attachment thereof to the two plate-shaped elements (U.S. Pat. No. 4,686,040) and filling of the space between the plate-shaped elements with hardening compound (U.S. Pat. Nos. 3,917,534 and 4,931,179). In the solution according to GB-A-894 318 heat presses are also required for laminating the plate-shaped elements.
In other the solutions mentioned, expensive forms are required for casting (DE-B-24 07 218), or moulding the plate shaped elements (U.S. Pat. No. 3,971,722), or moulding a complete filter sector (U.S. Pat. No. 3,917,534). Further, the casting and moulding procedures are time consuming.
The present invention has its first object to provide a new solution to the problem of making a filter sector of the kind discussed above, in which the plate-shaped elements are thin in comparison to the thickness of the sector, resistant to compression of the two plate-shaped elements and rigid to other deformation.
Another problem associated with filter sectors, in which filtrate is introduced through holes in the plate-shaped elements into the space between the two plate-shaped elements to be discharged through an outlet, is that the volume of the space often is so great that filtrate therein does not leave the filter sector before it has rotated so far that filtrate flows back out through the filter cloth and re-wets filter cake deposited thereon.
In the filter according to U.S. Pat. No. 4,686,040 this problem is solved in that a certain volume of the space is shut off from the remainder by one of the ribs mentioned or in that a compound fills said volume of the space.
In the filter according to U.S. Pat. No. 4,931,179, the hardened compound provides filling of the volume of the space between the plate-shaped elements which is not used for a radial channel for leading to the outlet of the filter sector.
The present invention has as its second object to provide a solution to the problem of making the volume of space between the plate-shaped elements of a filter sector as small as possible.